I have wondered about this too.
I have replaced mine with new spicer joints and use the red grease.

On the truck I am building I have one NOS Spicer 1610 series joint that says to use oil. I will buy two more new ones.
I have been looking for an explanation on why I can't use grease on all of them.

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-